What is the reason firewalls are considered stateful?

A. Firewalls keep track of the zone states.
B. Firewalls keep accounting on the state of packets.
C. Firewalls track the state of a TCP conversation.
D. Firewalls transition between defense states.

C. Firewalls keep track of the TCP conversation via the SYN-SYN/ACKACK three-way handshake. This is done so that a DoS attack such as a SYN flood can be mitigated.